This invention relates generally to mapping apparatus and methods, and more particularly concerns well mapping employing a probe which may be inserted into a bore-hole or well, for surveying and/or for tool steering. In addition, it concerns method and apparatus to determine the probe's degree of tilt from vertical and to relate the latter to sensor generated azimuth information, at all latitudes and at all instrument attitudes. Further, the azimuth determining apparatus by itself or in combination with the tilt measuring apparatus, may be housed in a carrier of sufficiently small diameter to permit insertion directly into available small I. D. drill tubing, thus eliminating the need to remove the tubing to enable such mapping.
In the past, the task of position mapping a well or bore-hole for azimuth in addition to tilt has been excessively complicated, very expensive, and often inaccurate because of the difficulty in accommodating the size and special requirements of the available instrumentation. For example, magnetic compass devices typically require that the drill tubing be fitted with a few tubular sections of non-magnetic material, either initially or when drill bits are changed. The magnetic compass device is inserted within this non-magnetic section and the entire drill stem run into the hole as measurements are made. These non-magnetic sections are much more expensive than standard steel drill stem, and their availability at the drill site must be pre-planned. The devices are very inaccurate where drilling goes through magnetic materials, and are unusable where casing has been installed.
Directional or free gyroscopes are deployed much as the magnetic compass devices and function by attempting to remember a pre-set direction in space as they are run in the hole. Their ability to initially align is limited and difficult, and their capability to remember degrades with time and environmental exposure. Also, their accuracy is reduced as instrument size is reduced, as for example becomes necessary for small well bores. Further, the range of tilt and azimuthal variations over which they can be used is restricted by gimbal freedom which must be limited to prevent gimbal lock and consequent gyro tumbling.
A major advance toward overcoming these problems is described in my U.S. Pat. No. 3,753,296. That invention provides a method and means for overcoming the above complications, problems, and limitations by employing that kind and principal of a gyroscope known as a rate-of-turn gyroscope, or commonly `a rate gyro`, to remotely determine a plane containing the earth's spin axis (azimuth) while inserted in a bore-hole or well. The rate gyroscope has a rotor defining a spin axis; and means to support the gyroscope for travel in a bore-hole and to rotate about an axis extending in the direction of the hole, the gyroscope characterized as producing an output which varies as a function of azimuth orientation of the gyroscope relative to the earth's spin axis. Such means typically includes a carrier containing the gyroscope and a motor, the carrier being sized for travel in the well, as for example within the drill tubing. Also, circuitry is operatively connected with the motor and carrier to produce an output signal indicating azimuthal orientation of the rotating gyroscope relative to the carrier, whereby that signal and the gyroscope output may be processed to determine azimuth orientation of the carrier and any other instrument thereon relative to the earth's spin axis, such instrument for example comprising a well logging device such as a radiometer, inclinometer, etc.
U.S. Pat. No. 4,199,869 improves upon U.S. Pat. No. 3,753,296 in that it provides for the obtaining of a very high degree of accuracy as respects derived azimuth and tilt information for all latitudes and angularities of bore-holes; the application of one or more two-degree of freedom gyroscopes as a "rate gyro" or rate gyros, for use in well mapping; the use of two such gyros in different attitudes to obtain cross-check azimuth information; and the provision of highly compact instrumentation which is especially needed for smaller diameter bore-holes.
While the devices of the above two patents are highly useful, they lack the unusual features and advantages of the present invention, among which are: compensation for certain errors in the outputs of the angular rate sensor or sensors, and in the outputs of the tilt or acceleration sensor or sensors. Typical of such errors are bias errors, acceleration sensitive errors and acceleration squared errors, and temperature and time induced errors.